I've tried a lot of stuff to do the wheels. Nothing looked good. I even tried to modify bottle tops. But after taking a look at my old toys I found a model plane with these. They aren't as orcish as I would have liked them to be, but quite cool anyway.

Maybe you could add some chain to the tyres or small spikes.
Fine chain is easy to get hold of just go and take some from your mums or girlfriends jewlery box. Small spikes can be made from cutting the tip of cocktail sticks.

The model looks really great. The thin bike looks really 'cool', much better than the bulky original ones, I'd say. ^_^ I'd follow Boomer's advice and make some cocktail stick spikes for the wheels, would make them look more Orky.

But overall, very good job. What an awesome, thoughtful gift for your friend. ^___^